Hemodialysis, as practiced today, normally employs one of two types of catheter to remove blood from the patient for processing and return processed blood to the patient. Most commonly, a catheter tube containing two lumens is used, each lumen having a generally semi-cylindrical or D-shape configuration. This type of catheter is frequently referred to as a dual lumen catheter. Alternatively, two tubes, each with a full cylindrical configuration, are used separately to remove blood for dialysis and return the processed blood.
Flow rates possible with conventional dual lumen catheters are usually lower than those achievable where separate tubes are used to remove blood from a vein for dialysis and then return processed blood back to the vein. Thus, the use of two tubes has become more and more popular as the capacity (maximum flow rate) of hemodialysis membranes has increased. However catheters utilizing two separate lumens are more difficult to insert and also take up considerably more space in the vessel, thereby somewhat compromising blood flow in the vessel.
Hemodialysis membranes are now able to process blood at over 500 ml of flow per minute. Even higher processing rates are foreseeable. However, problems occur with both the line introducing purified blood back into the vein (the venous or outflow line) and the line removing blood for purification (the arterial or intake line) at flow rates above 300 ml per minute. A high flow rate from the venous line may cause whipping or “fire-hosing” of the tip in the vein with consequent damage to the vein lining. A corresponding high flow rate into the arterial line may cause the port to be sucked into the vein wall, resulting in occlusion. It should be understood, of course, that both lines normally access the superior vena cava and the left atrium and the designations are used for differentiation purposes.
Speed of flow through a catheter lumen, whether it be in a single lumen or a dual lumen catheter, is controlled by a number of factors including the smoothness of the wall surface, the internal diameter or cross-sectional area of the tube lumen, and the length of the tube lumen. The most important factor is the cross-sectional area of the tube lumen. The force or speed of the fluid flow in a tube lumen for a given cross-sectional area is controlled by the external pumping force, of course. This is a positive pressure pushing processed blood through the venous lumen and a negative (suction) pressure pulling unprocessed blood through the arterial lumen.
Problems encountered in providing for a high flow rate through a catheter are magnified in a dual lumen catheter construction. Because each of the lumens in a dual lumen catheter normally has a D-shape, it has been assumed that flow rates are limited. Furthermore, such dual lumen catheters are, to a great extent, catheters with a main port that opens at the end of a lumen substantially on the axis of the lumen. Thus, “fire-hosing” frequently results. Fire-hosing may damage the vein wall, triggering the build-up of fibrin on the catheter tip. Fibrin build-up may result in port occlusion.
There are dual lumen catheters described in the prior art which utilize side ports for both outflow and inflow. An example is the catheter disclosed in the Cruz et al. U.S. Pat. No. 5,571,093. Another example is the catheter disclosed in Quinn U.S. Pat. No. 6,461,321. Yet another example is the catheter disclosed in the DeCant, Jr. et al. U.S. Pat. No. 6,786,884. Each of these catheter designs combines a dual lumen catheter tube with a dual passage bolus, which is independently formed and then attached to the distal end of the tube. Each has unique shortcomings in performance and/or manufacturability which detract from its attractiveness as a commercial product, however.